The present invention relates to a method of producing a body for electric heating by compaction and densification, or consolidation, of pulverulent, metallic material. The present invention also relates to a body for heating purposes comprising metal powder and produced in accordance with the method.
Bodies for heating purposes, for example different types of sealing or welding jaws are regularly employed within a number of different branches of industry. Bodies for electric heating of different types of materials are thus employed inter alia within the packaging industry, in particular in the production of packaging containers of packaging laminates which consist of outer layers of thermoplastic, i.e. thermosealable material. Packaging laminates of this type are employed, for example, for producing packaging containers for various types of beverages, for example juice or milk. These packaging containers are created by folding and thermosealing of the thermoplastic material. The thermosealing procedure is such that two or more layers of the packaging laminate within the desired sealing zone are urged against one another with the aid of sealing jaws, of which at least one includes an electric conductor which makes for heating and thermosealing of the material. In connection with the compression of the packaging laminate layers, the sealing jaw is activated so that the mutually abutting, exteriorly thermoplastic layers are heated to softening temperature and partly fuse together. After a certain cooling time, the laminate layers within the sealing zone are sealed to one another in a durable and liquid-tight sealing joint or seam, and the sealing jaws may once again be distanced from one another so that the sealed laminate may be removed and replaced by new, as yet unsealed laminate portions. In packing and filling machines of modern design, the production of packaging containers takes place at high speed, and the resultant necessary high sealing rate renders it difficult to ensure satisfactory cooling of the sealing jaws in the time interval between two sequential sealing operations. In order to prevent the sealing jaws from becoming so hot that they prevent efficient cooling of the fused thermoplastic layers before the sealing jaws are distanced from one another, a cooling of the sealing jaws is therefore necessary. Normally, this takes place in that the sealing jaws are, in addition to the electrically conductive heating strip or coil, also provided with ducts for coolant fluid, normally water. Naturally, this renders both the manufacture of the sealing jaws proper and the overall packaging and filling machine more complicated.
The sealing jaw for electric heating in connection with, for example, the sealing of thermoplastic packaging materials is produced in a number of different ways, in a plurality of different configurations and different designs and constructions. Requirements common to sealing jaws of this type are that they must display high form precision, high durability to loading and good thermal conductivity. Since, moreover, it is also often required that the sealing jaw displays a relatively complicated configuration, sealing jaws are nowadays often produced using modern methods and techniques, for example by compacting powder metal material and subsequent sintering. This technique is per se well known and briefly comprises filling of powder metallic material into a suitable press mould, compaction of the material to the desired extent and subsequent heat treatment or sintering in order to cause the powder metal material to bond together and form a solid and configurationally stable body. The conductor necessary for electric heating may be placed in the mould before the metal powder and itself consist of powder metal material in the form of uninsulated metal particles of, for example copper, or be placed in a groove provided in the body of the sealing jaw. For further information within this area, reference is made to European Patent Specification EP 687.215.
In the production of bodies of pulverulent, metallic material, a compaction of the metal powder thus takes place in the mould. However, despite extremely high pressure it is impossible to compact the material to such an extent that the pulverulent metal particles wholly fuse together or are caused to totally fill out the space provided in the mould. In actual fact, there consequently always occur larger or smaller air-filled pockets or pores between the particles which, when the produced body is to be employed as a part of, for example, a thermosealing jaw, has proved to be a major disadvantage since the entrapped air serves as insulation and prevents the dissipation of heat through the body. As a result, the removal of excess heat and the emission of heat into the ambient surroundings are also prevented, and sealing jaws with bodies of powder metallic material therefore show a tendency, in lengthy intensive use, to be heated to a higher level than bodies in sealing jaws which are manufactured from a more homogeneous material. In this context, reference is made to PCT WO 97/14547 for further information. A known technique takes as its point of departure replacing the air-filled pockets with a plastic material (epoxy polymer), but this technique has only limited fields of use and is less suitable in connection with bodies for thermosealing jaws, since the plastic does not withstand the relatively high temperatures to which such a body is exposed in connection with continuous operation in a packing and filling machine.
There is thus a general need within the art disclosed by way of introduction to realise a method which makes for the production of a body of pulverulent, metallic material, the body having its pores located between the pulverulent metal particles filled with a thermally conductive and heat-resistant material.
One object of the present invention is to realise a method of producing a body for electric heating from pulverulent, metallic material, the method making it possible to fill out the pores occurring between the pulverulent metal particles with a thermally conductive, heat-resistant, hard and configurationally resistant material.
A further object of the present invention is to realise a method of the above-outlined type which makes it possible to select to degree to which the pores are filled.
Yet a further object of the present invention is to realise a method of the above-outlined type, the method being capable of being employed in the production of bodies for electric heating, i.e. bodies which contain prefabricated parts or space for such parts.
Still a further object of the present invention is to realise a method of the type indicated by way of introduction which makes it possible, in an integrated and efficient manner, to provide a body produced from metal powder with a homogeneous densification or consolidation material with properties which are particularly suitable for use in sealing jaws for electric heating.
The above and other objects have been attained according to the present invention in that a method of the type disclosed by way of introduction has been given the characterizing features that the pulverulent material is fed into a mould and compacted, that also an impregnating solution capable of being converted into a metal oxide is added, and that the body is thereafter heated to a temperature requisite for drying the impregnating solution and bonding the oxide to the pulverulent, metallic material.
Preferred embodiments of the method according to the present invention have further been given the characterizing features as set forth in appended subclaims 2 to 13.
The present invention also has for its object to realise a body for heating purposes, the body including, in accordance with the method according to the present invention, metal powder material possessing superior thermal conductivity.
A further object of the present invention is to realise a body for heating purposes, the body possessing superior thermal conductivity and such configurational consolidation and strength that it is well-suited for use in sealing jaws intended for electric heating.
Yet a further object of the present invention is to realise a body of the above-outlined type which possesses electrically insulating properties.
Still a further object of the present invention is to realise a body of the above-outlined type, the body accommodating an electrically conductive part.
The above and other objects have been attained according to the present invention in that a body of the type disclosed by way of introduction has been given the characterizing feature that it includes pulverulent, metallic material of iron of a particle size of less than 300 xcexcm, and a ceramic material located between the particles.
Preferred embodiments of the body according to the present invention have further been given the characterizing features as set forth in appended subclaims 15 to 21.
As a result of the method and the body according to the present invention, a method and an apparatus will be realised which, in a rational manner, makes it possible to produce bodies for use, for example, in thermosealing jaws, the bodies possessing superior conductivity for heat, but poor conductivity for current. Hereby, the problem of accumulated heating which has hitherto created difficulties in rapid, repeated thermosealing in modem packing and filling machines is obviated. The production method makes it possible to fill-out, to the desired degree, pores in the body with a ceramic, thermally conductive material. The method further makes it possible to provide the body with an electric conductor or coil during a desired phase in the production process. After production, the body obtains the desired configurational accuracy and configurational stability and may be optionally subjected to further processing or mounting of additional, prefabricated parts.